Chapter 7

Intertonguing of the Lower Part of the 

Uinta Formation with the Upper Part 

of the Green River Formation in the 

Piceance Creek Basin During the Late 

Stages of Lake Uinta

By John R. Donnell

Chapter 7 of 7

Oil Shale Resources of the Uinta Basin, Utah and Colorado 

By U.S. Geological Survey Oil Shale Assessment Team

U.S. Geological Survey Digital Data Series DDS–69–BB

U.S. Department of the Interior

U.S. Geological Survey

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Volume Title Page
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Donnell, J.R., 2010, Intertonguing of the lower part of the Uinta Formation with the upper part of the Green River 

Formation in the Piceance Creek Basin during the late stages of Lake Uinta: U.S. Geological Survey Digital Data 

Series DDS 69–BB, chp. 7, 25 p. Also may be found as a Scientific Investigations Report (http://pubs.usgs.gov/

sir/2008/5237/).
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Previous Work ................................................................................................................................................3

Present Investigation ....................................................................................................................................3

Subsurface Studies ..............................................................................................................................3

Detailed Mapping..................................................................................................................................3

Delineation of Tongues ........................................................................................................................3

Green River Tongues ...................................................................................................................3

Uinta Tongues...............................................................................................................................8

Description of Individual Tongues in the Green River and Uinta Formations ......................................8

Marlstone at Calamity Ridge...............................................................................................................8

Marlstone at Greasewood Creek.......................................................................................................8

Marlstone at Trail Canyon ...................................................................................................................8

Marlstone at Mare Canyon .................................................................................................................8

Yellow Creek Tongue ............................................................................................................................9

Dry Fork Tongue ....................................................................................................................................9

Thirteenmile Creek Tongue ...............................................................................................................23

Black Sulphur Tongue ........................................................................................................................23

Coughs Creek Tongue ........................................................................................................................23

Stewart Gulch Tongue........................................................................................................................23

Marlstone at Barnes Ridge and Marker Bed at Bull Fork............................................................24

Marlstone at Sleepy Ridge and Marlstone at Jackrabbit Ridge.................................................24

Marlstone at Skinner Ridge ..............................................................................................................24

Shale-Oil Resources....................................................................................................................................24

References Cited..........................................................................................................................................25

Figures

1. Map showing location of the Piceance Creek Basin study area .........................................2

2. Map showing location of data used in North-South cross section .....................................4

3. North-South cross section depicting the encroachment of fluvial sediments

 of the Uinta Formation into a 500 foot thickness of the Green River

 Formation .......................................................................................................................................5

4. Cross section showing the correlation of individual numbered oil shale 

 beds and groups of beds in the upper part of the Green River Formation..........................6

5. Published geologic quadrangle maps in the area of this report ..........................................7

6. Map showing location of cross sections A–A′, C–C′, E–E′, G–G′, 

 and I–I′ .........................................................................................................................................11

7. Cross section A–A′ showing Green River Tongue Tggc.....................................................12

8. Map showing location of data used in cross sections B–B′, D–D′, F–F ′, 

 H–H′, and J–J′............................................................................................................................13
iv

9. Cross section B–B′ showing Green River Tongue Tgtc .....................................................14

10. Cross section C–C′ showing Green River Tongue Tgm .....................................................15

11. Cross section D–D′ showing Green River Tongue Tgy.......................................................16

12. Cross section E–E ′ showing Green River Tongue Tgt.........................................................17

13. Cross section F–F ′ showing Green River Tongue Tgb........................................................18

14. Cross section G–G′ showing Green River Tongue Tgc.......................................................19

15. Cross section H–H′ showing Green River Tongue Tgs.......................................................20

16. Cross section I–I′ showing Green River Tongue Tgj...........................................................21

17. Cross section J–J ′ showing Green River Tongue Tgsk......................................................22

Tables

1. Mapped Green River Formation tongues in the Piceance Creek Basin, 

northwestern Colorado................................................................................................................9

2. List of drill holes shown on cross sections ............................................................................10
Abstract

During most of middle Eocene time, a 1,500-mi2 area 

between the Colorado and White Rivers in northwestern Colorado was occupied by the Piceance lobe of Lake Uinta. This 

initially freshwater lake became increasingly saline throughout its history. Sediments accumulating in the lake produced 

mostly clay shale, limestone, and dolomite containing varying 

concentrations of organic matter.

At the time of the maximum extent of the lake, the 

organic-rich Mahogany bed of the Green River Formation 

was deposited throughout the area. Shortly after its deposition, stream deposits began infilling the lake from the north 

through a series of contractions interspersed with minor 

expansions. This fluctuation of the shoreline resulted in the 

intertonguing of the stream sediments of the lower part of the 

overlying Uinta Formation with the lacustrine sediments of the 

upper part of the Green River over a distance of about 40 mi; 

construction of regional stratigraphic cross sections show the 

pattern of intertonguing in considerable detail.

The data utilized in this study, which covered parts of 

Rio Blanco, Garfield, and Mesa counties, were derived from 

(1) geologic mapping of thirty-four 71⁄2-minute quadrangles 

and stratigraphic studies by geologists of the U.S. Geological 

Survey, and (2) shale-oil assay information from numerous 

cores. As a result of this previous work and the additional 

effort involved in the compilation here presented, more than a 

dozen Green River Formation tongues have been named, some 

formally, others informally.

Middle Eocene strata above the Mahogany bed in the 

northern part of the study area are dominantly coarse clastics 

of the Uinta Formation. The sedimentary sequence becomes 

more calcareous and organic-rich to the south where, in a 

400-mi2

 area, a 250-ft-thick sequence of oil shale above the 

Mahogany bed contains an average of 16 gallons of oil per ton 

of shale and is estimated to contain 73 billion barrels of oil.

Introduction

The Piceance Creek Basin is a large structural and depositional basin of several thousand square miles in northwestern 

Colorado, extending from the Axial uplift in the north to the 

Gunnison uplift in the south and from the White River uplift 

on the east to the Douglas Creek arch to the west. During a 

period of several million years in the middle Eocene, most of 

the basin was occupied by the Piceance lobe of Lake Uinta, 

a large body of water that covered much of northwestern 

Colorado and northeastern Utah (Franczyk and others, 1992). 

Throughout its history, coarse clastics were shed into the marginal areas of the lake while organic-rich material that formed 

the oil shale beds of the Green River Formation was deposited 

in the open lacustrine area. A sequence of rich oil shale beds 

named the “Mahogany Ledge” were deposited during the time 

of maximum extent of the lake. The Mahogany bed in the 

upper third of that sequence is about 10 ft thick and in much of 

the area yields an average of 50 gallons of oil per ton of shale. 

The unit is resistant to erosion and forms a prominent ledge 

in exposures along the south margin of the basin. The coarse 

clastic beds that underlie and intertongue with the oil shales 

below the Mahogany bed are in the Wasatch Formation and 

those that overlie and intertongue with the oil shales above the 

Mahogany bed are in the Uinta Formation.

Concern about a possible shortage of petroleum products 

in the United States in the 1940s created considerable interest 

in possible utilization of oil from oil shale to help meet the 

nation’s energy needs. The Federal Government and major 

oil companies initiated a core-hole drilling program and the 

U.S. Geological Survey (USGS) initiated a detailed geological 

mapping program in a 1,500-mi2

 area of the Piceance Creek 

Basin (fig. 1) between the Colorado River on the south and the 

White River on the north. Assays of the oil shale from several 

hundred cores combined with the mapping data provided 

the information needed to prepare a comprehensive shale-oil 

Intertonguing of the Lower Part of the Uinta Formation 

with the Upper Part of the Green River Formation in the 

Piceance Creek Basin during the Late Stages of Lake 

Uinta

By John R. Donnell
2 Intertonguing in the Piceance Creek Basin During the Late Stages of Lake Uinta

70

White River

108° 30' 108°00'

40°00'

39° 30'

R. 102 W. R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W. R. 94 W.

Highway

12 MILES

City or town

County line

Creek or river

0

Area underlain by

 the Mahogany bed

EXPLANATION

Colorado

64

139 64

13 White River

Meeker

Rangely

Piceance

Rio Blanco

Creek

GARFIELD

RIO BLANCO

139

13

Rifle

DeBeque

Parachute

Colorado River

Creek

Mesa

Roan

Parachute Creek

T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

T. 4 S.

T. 3 S.

T. 2 S.

T. 1 S.

T. 1 N.

T. 2 N.

R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W.

Index map

Figure 1. Location of the Piceance Creek Basin study area between the Colorado and the White Rivers in northwestern 

Colorado.
Present Investigation 3

resource appraisal of the Piceance Creek Basin (Pitman and 

Johnson, 1978; Pitman and others, 1989). 

Previous Work

The extensive core-hole drilling and detailed mapping 

delineated the complex intertonguing of the upper part of 

the Green River Formation with the lower part of the Uinta 

Formation in the Piceance Creek Basin. Most of the prominent 

Green River tongues were formally named, whereas the Uinta 

tongues received no formal designation but were labeled either 

by letter or number varying from quadrangle to quadrangle. 

The type localities of most tongues are a considerable distance 

from the area where they join the main body of the Green 

River Formation. Therefore, the identification of individual 

organic-rich oil shale beds in a given tongue at its type locality 

is difficult because of the dilution of organic fraction by clastic 

sediments.

Surface and subsurface data indicate that relatively soon 

after the deposition of the Mahogany bed, the Piceance lobe of 

Lake Uinta began to infill from the north with clastic material 

of the Uinta Formation. The lowermost Uinta tongue is less 

than 10 ft above the Mahogany bed in a small area located in 

Ts. 1 and 2 N., and Rs. 99 and 100 W. in the northwest corner 

of the basin (fig. 1). As infilling progressed from the north, 

lacustrine deposition continued in the southern part of the 

basin. Throughout much of the period of infilling by clastic 

sediments in the north, a continuous sequence of oil shale 

more than 400 ft thick was deposited above the Mahogany bed 

in a northwest-trending area from T. 7 S., R. 94 W. to T. 6 S., 

R. 99 W. in the southern part of the basin (fig. 1). A number of 

cross sections have been published showing the intertonguing 

throughout the basin, utilizing both surface and subsurface 

data; however, none makes a precise tie between specific Uinta 

tongues and individual persistent organic-rich oil shale beds 

(O’Sullivan, 1974; 1987; Hail, 1987; 1990; 1992). 

Present Investigation

The present study utilized the data from detailed mapping 

and stratigraphic studies, as well as shale-oil assay data from 

numerous cores in the Piceance Creek Basin to show a direct 

tie between identifiable rich oil shale beds in the upper part 

of the Parachute Creek Member of the Green River Formation with individual clastic beds or groups of beds in the lower 

part of the Uinta Formation (figs. 2 and 3). The assumption is 

that zones in cores that have little or no shale-oil content are 

siltstones or sandstones of the Uinta Formation (as shown by 

assay values in fig. 3); several such zones in core holes C 34 

and C 35 can be correlated with oil shale tongues in adjacent 

core holes, as indicated. Identification of individual or groups 

of oil shale beds contained in a Green River tongue is based 

on their comparison with numbered beds shown in figure 4. 

Although several hundred sets of assays are available, there 

are some areas with no data. Positive identification of individual oil shale beds is difficult where assay intervals are 

greater than two feet. Uinta tongues consist of clastic beds that 

underlie and intertongue with the laterally equivalent tongues 

of the Green River Formation; tongues of the Uinta Formation 

are numbered from oldest to youngest (fig. 4).

Subsurface Studies

Assay data from selected cores in boreholes throughout 

the Piceance Creek Basin were examined to determine the 

relation of each Uinta tongue to the numbered oil shale beds 

shown in figure 4. Cross sections of individual tongues were 

prepared using assay sets from holes drilled close to where a 

given Green River tongue joins the main body of the formation; these sets were then correlated with other sets at some 

distance from the join. Positive identification of individual oil 

shale beds is difficult in core holes with large assay intervals, 

therefore only wells with one- or two-foot assay intervals were 

used. Individual Uinta tongues in the cross sections include 

sandstone and siltstone beds that underlie and intertongue with 

the Green River.

Detailed Mapping

Geologic mapping was initiated in 1960 in the first of 

34 USGS 1:24,000-scale 71⁄2-minute quadrangle maps in the 

Piceance Creek Basin (fig. 5), and continued until the late 

1980s. Initially, individual tongues of the Green River Formation were not differentiated in outcrops where they were interbedded with units consisting of Uinta Formation lithologies, 

therefore the tongues were not mapped on many of the earliest 

published geologic quadrangles. During the present investigation, in areas where tongues are mapped to the margin of one 

quadrangle, however, they are extended into adjacent quadrangles where they were not initially mapped. In some areas close 

to the merger of a Green River tongue with the main body of 

the formation, the associated Uinta tongue is too obscured to 

be mapped; however, the tongue’s presence can be determined 

based on assays of core samples. 

Delineation of Tongues

Green River Tongues

The Green River tongues are characterized in much of 

the Piceance Creek Basin by prominent light-gray to white 

bands separated by thick, brown, slope-forming sandstones of 

the Uinta Formation along Piceance Creek and Yellow Creek 

and their major tributaries. The lithology at most type localities consists dominantly of light-gray to white marlstone, with 

little or no organic content, and minor amounts of siltstone 
4 Intertonguing in the Piceance Creek Basin During the Late Stages of Lake Uinta

70

White River

108°30' 108°00'

40°00'

39°30'

R. 102 W. R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W. R. 94 W.

12 MILES

North-south cross

 section

0

Colorado

64

139 64

13 White River

Meeker

Rangely

Piceance

Rio Blanco

Creek

GARFIELD

RIO BLANCO

139

13

Rifle

DeBeque

Parachute

Colorado River

Creek

Mesa

Roan

Parachute Creek

T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

T. 4 S.

T. 3 S.

T. 2 S.

T. 1 S.

T. 1 N.

T. 2 N.

R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W.

Index map

C34

C35

C254

C333

C37

Core hole

Highway

City or town

County line

Creek or river

Area underlain by

 the Mahogany bed

EXPLANATION

Figure 2. Location of data used in north-south cross section.
350400450500550600650700750800850900950

1,0001,050

1,100

1,050

1,000

950900850

800

750700650

600

550

500

450

400

350

300

800

850900950

1,000

1,050

1,1001,150

1,200

1,2501,300

1,3001,250

1,150

1,000

950900850800750700650600550500

60

58

54

52

44

40

38

322622

1812

MB

0 20 40 60 80

0 20 40 60 80

0 20 40 60 80 0 20 40 60 80 

?

???

?

1,050

1,100

1,200

66 ?64 ?

A Groove

Assay values, in gallons per ton

Assay values, in gallons per ton

Assay values, in gallons per ton Assay values, in gallons per ton 

Core Hole

C333

Getty Oil Co., 32–27

Sec. 27, T. 5 S., R. 97 W.

Core Hole

C254

Arco (C–b Project), Sorghum Gulch 11

Sec. 7, T. 3 S., R. 96 W.

Core Hole

C35

U.S. Bureau of Mines/AEC, Colorado 2

Sec. 14, T. 1 S., R. 99 W.

Core Hole

C34

U.S. Bureau of Mines/Aec, Colorado 1

Sec. 13, T. 1 N., R. 98 W.

Mahogany Ledge

Mahogany Marker

DEPTH, IN

FEET

DEPTH, IN

FEET

DEPTH, IN

FEET

DEPTH, IN 

FEET

Figure 3. North-South cross section depicting the encroachment of fluvial sediments of the Uinta Formation into a 500 foot thickness of the Green 

Note: For larger version, click here.
6 Intertonguing in the Piceance Creek Basin During the Late Stages of Lake Uinta

0

50

100

150

200

250

300

350

400

150

200

250

300

350

400

450

500

550

600

U31

 Skyline Oil Company

 Watson 1

Sec. 5, T. 11 S., R. 25 E.

 Uintah County, Utah

0 20 40 60

0 20 40 60

C37

U.S. Bureau of Land Management

 Triangulation Station Shale 2

 Sec. 24, T. 7 S., R. 97 W.

 Garfield County, Colo.

74

72

70

68

66

64

62

60

58

56

54

52

50

48

46

44

42

40

38

36

34

32

30

28

26

24

22

25

18

16

14

12

10

8

6

4

2

Tgsk

Tgsl, Tgj

Tgbr

Tgs

Tgc

Tgb

Tgt

Tgd

Tgy

Tgm

Tgtc

Tggc

Tu 13

Tu12

Tu11

Tu10

Tu9

Tu8

Tu7

Tu6

Tu5

Tu4

Tu3

“A” Tu2

76

74

72

70

68

67

66

64

62

60

58

56

54

52

50

48

46

44

42

40

39

38

37

34

32

30

28

26

25

24

22

20

18

16

14

12

10

8

6

4

2

36

20

Oil yield, in gallons per ton

Oil yield, in gallons per ton

DEPTH, IN

FEET

DEPTH, IN

FEET

67

37

35

21

39

35

21

Figure 4. Cross section showing correlation of individual oil shale beds (numbered 2–76) 

and groups of beds forming named tongues (indicated by stratigraphic unit symbols) in the 

upper part of the Green River Formation. Stratigraphic units labeled “Tu” are tongues of Uinta 

Formation. Symbols for beds and tongues are defined in table 1. From Donnell, 1997.
Present Investigation 7

Rough

Gulch

Smizer

Gulch

Barcus

Creek

 Wolf 

Ridge

Yankee

 Gulch

Figure

 Four

Spring

Desert

 Gulch

 The

Saddle

Calamity

 Ridge

Sagebrush

 Hill

Black

Cabin

Gulch

Razor

 Back

Ridge

Henderson

 Ridge

Barcus

 Creek

 SE

Square S

 Ranch

 Rock

School

Bull

Fork

Mount

Blaine

Long

Point

White

River

 City

Greasewood

 Gulch

Jessup

 Gulch

Cutoff

Gulch

Circle

 Dot

Gulch

 Red

Pinnacle

 Segar

Mountain

 No

Name

Ridge

McCarthy

 Gulch

Forked

 Gulch

Grand

Valley

Thirteen

 Mile

 Creek

 Rio

Blanco

 Anvil

Points

Rulison

Meeker

Top of Mahogany Ledge

Figure 5. Locations of published U.S. Geological Survey 1:24,000-scale 71⁄2-minute quadrangle maps in the Piceance Creek 

Basin, northwestern Colorado.
8 Intertonguing in the Piceance Creek Basin During the Late Stages of Lake Uinta

and sandstone, algal and oolitic limestone, and, locally, thin 

beds of oil shale. Traced to the south from the type localities, 

the tongues generally contain a higher percentage of organic 

material and join the main body of the Green River Formation 

where they contain identifiable rich oil shale beds. Traced to 

the north from the type localities, the tongues generally have 

a higher percent of sandstone and siltstone as they merge into 

the Uinta Formation.

Uinta Tongues

At the type locality of each Green River tongue, the 

immediately underlying Uinta tongue is assigned a number 

(see figs. 3 and 4) and described in the text. South of the type 

locality of the Green River tongue, the sandstone of the underlying Uinta tongue thins and wedges out, whereas north of the 

type locality of the Green River tongue, the underlying Uinta 

sandstone tongue thickens and merges with the next higher 

Uinta tongue.

Description of Individual Tongues in 

the Green River and Uinta Formations

Table 1 lists the particular publication wherein a given 

tongue is named and described in detail, and table 2 lists the 

drill holes shown on the cross sections. This report describes 

the relation of each Green River and Uinta tongue according 

to the numbered beds and named units shown in figures 3 and 

4. The geographic extent of each tongue is estimated based on 

detailed surface mapping within the quadrangle areas shown 

in figure 5 and on subsurface data derived from assays of core 

plotted on a series of cross sections (A–A′ to J–J ′) presented 

in figures 7, and 9 through 17. Locations of cross sections are 

shown in figures 6 and 8.

Marlstone at Calamity Ridge

The lowermost oil shale tongue of the Green River 

Formation, here named informally as the “Marlstone at 

Calamity Ridge” (Tgcr, fig. 3) lies above the Mahogany 

bed. It is entirely within the upper part of a unit termed the 

“Mahogany Ledge.” The tongue extends from the “Mahogany 

marker” at its base, which is a thin analcimized tuff bed 

above the Mahogany bed, upward to the base of “A-groove” 

that is the upper contact of the Mahogany Ledge. The lowest 

tongue of the Uinta Formation (distinguished as Tu1; it is not 

shown in figures or tables) consists of a basal sandstone at the 

“Mahogany Marker” and two other sandstone beds higher in 

the section. The Tgcr and Tu1 unit appear to be restricted to 

a small area in the north half of the Calamity Ridge, Barcus 

Creek, and Barcus Creek SE quadrangles (fig. 5) and have 

been mapped as part of the Parachute Creek Member of the 

Green River Formation.

Marlstone at Greasewood Creek

The “Marlstone at Greasewood Creek,” near the area 

where it joins the main body of the Green River Formation, 

contains oil shale beds 2 through 12 that form tongue Tggc

(fig. 4; see cross section A–A′; figs. 6 and 7). A persistent 

sandstone in “A-groove” is the basal unit in underlying Uinta 

tongue Tu2. The sandstone that directly overlies tongue 

Tggc is probably the lateral equivalent of the wavy-bedded 

tuff in the southern part of the Piceance Creek Basin. Tongue 

Tggc may be traced in the subsurface into the White River 

City quadrangle, the northeastern part of the Square S Ranch 

quadrangle, most of the Sagebrush Hill quadrangle, and the 

southern third of the Wolf Ridge quadrangle (fig. 5). It merges 

with the Uinta Formation in the southwest quarter of the 

Barcus Creek SE quadrangle and the northwest quarter of the 

Wolf Ridge quadrangle, and joins the main body of the Green 

River Formation on the southern edge of the Sagebrush Hill 

and Wolf Ridge quadrangles and the central part of the Square 

S Ranch quadrangle (fig. 5). 

Marlstone at Trail Canyon 

The Green River tongue distinguished as the “Marlstone 

at Trail Canyon” (Tgtc, fig. 4; cross section B–B′; figs. 8 and 

9) was informally named by Hail (1984) for exposures along 

Trail Canyon in the Calamity Ridge quadrangle. At the type 

locality it is mostly light-gray marlstone with lesser amounts 

of siltstone. The tongue has been mapped in the Barcus Creek, 

Barcus Creek SE, and the Calamity Ridge quadrangles and 

traced in the subsurface into the northern part of the Wolf 

Ridge quadrangle (fig. 5). It joins the main body of the Green 

River Formation in the southern part of the Calamity Ridge 

quadrangle and grades into Uinta Formation tongue Tu3 in 

the northwest quarter of the Barcus Creek quadrangle. Tongue 

Tgtc contains oil shale beds 16, 18, and 20 (fig. 4). The basal 

sandstone in Tu3 is the lateral equivalent to a unit called the 

“wavy bed,” a prominent tuff bed in the southern part of the 

basin. Tu3 has been labeled Tub, Tu2, and Tucb on various 

published quadrangles.

Marlstone at Mare Canyon 

The tongue “Marlstone at Mare Canyon” (Tgm, fig. 4; 

cross section C–C′, figs. 6 and 10) was informally named 

by Hail (1984) for exposures in the NW1⁄4 sec. 34, T. 1 N., 

R. 99 W., along Trail Canyon just east of the mouth of Mare 

Canyon. At the type locality, it consists mostly of light-gray 

silty marlstone and marly siltstone. In the vicinity of its join 

with the main body of the Green River Formation, tongue 

Tgm contains rich oil shale beds 22, 24, and 25. In places, a 

sandstone bed as much as 40-ft thick separates beds 24 and 

25 (fig. 10). The unit has been mapped in the Barcus Creek, 

Barcus Creek SE, and Calamity Ridge quadrangles (fig. 5); 

in the Sagebrush Hill quadrangle it has been mapped by 
Description of Individual Tongues in the Green River and Uinta Formations 9

Table 1. Mapped Green River Formation tongues in the Piceance Creek Basin, northwestern Colorado.

[USGS, U.S. Geological Survey; MF, Miscellaneous Field Studies Maps; Bull., Bulletin; GQ, Geologic Quadrangle Maps] 

Name Map 

symbol

Status of 

name

Underlying

Uinta Tongue Publication

Marlstone at Skinner Ridge Tgsk Informal Tu13 USGS Map MF–984 (Hail, 1978)

Marlstone at Sleepy Ridge Tgsl Informal Tu12 USGS Map MF–984 (Hail, 1978)

Marlstone at Jackrabbit Ridge Tgj Informal Tu11 USGS Map MF–1293 (Hail, 1984)

Stewart Gulch Tongue Tgs Formal Tu10 USGS Bull. 1422–E (Hail, 1977)

Coughs Creek Tongue Tgc Formal Tu9 USGS Bull. 1395 (O’Sullivan, 1975)

Black Sulphur Tongue Tgb Formal Tu8 USGS Bull. 1394–F (Duncan and others, 1974) 

Thirteen Mile Creek Tongue Tgt Formal Tu7 USGS Bull. 1394–F (Duncan and others, 1974)

Dry Fork Tongue Tgd Formal Tu6 USGS Bull. 1394–F (Duncan and others, 1974)

Yellow Creek Tongue Tgy Formal Tu5 USGS Bull. 1394–F (Duncan and others, 1974)

Marlstone at Mare Canyon Tgm Informal Tu4 USGS GQ 1578 (Hail, 1984)

Marlstone at Trail Canyon Tgtc Informal Tu3 USGS GQ 1578 (Hail, 1984)

Marlstone at Greasewood Creek Tggc Informal Tu2 This report

Marlstone at Calamity Ridge Tgcr Informal Tu1 This report

Donnell (1982) as Tgdc. In the subsurface, the tongue has 

been identified in the Wolf Ridge quadrangle in core hole C 35 

(fig. 10). The “Marlstone at Mare Canyon” in the northeastern 

part of the Barcus Creek SE quadrangle has been mapped 

as tongues Tg3 and Tg4, the lateral equivalent of Uinta 

tongues Tu3 and Tu4 which are combined in this report as 

Tu4 (fig. 10). The unit has not been mapped separately in the 

White River City quadrangle nor identified in the subsurface, 

probably because it has graded into the Uinta Formation.

Yellow Creek Tongue

The Yellow Creek Tongue (Tgy, fig. 4; cross section 

D–D′, figs. 8 and 11) was named by Duncan and others (1974) 

from exposures on the east side of Yellow Creek in the NE1⁄4 

SE1⁄4 sec. 13, T. 1 N., R. 98 W. about 150 ft southeast of U.S. 

Bureau of Mines AEC core hole 1. At the type locality it is 

45 ft thick and comprised mainly of light-gray to mediumbrown, silty marlstone with minor, thin interbeds of light-gray 

to light-brownish gray beds of siltstone and a one-foot-thick 

medium-grained sandstone bed. Near where the tongue merges 

with the main body of the Green River Formation, it consists 

almost entirely of oil shale, including the four organic-rich 

beds (nos. 26, 28, 30, and 32; figs. 4 and 11) that have been 

termed “the 4 senators” in the area just north of the Colorado 

River (fig. 1). Tongue Tgy has been mapped in the White 

River City, Barcus Creek, Barcus Creek SE, and Greasewood 

Gulch quadrangles (fig. 5); it has also been recognized in the 

southern part of the Rough Gulch quadrangle, as well as in 

the subsurface of the Wolf Ridge, Square S Ranch, and Black 

Cabin Gulch quadrangles. The magnitude of the intertonguing 

between the Green River and Uinta Formations in the basin 

is demonstrated by the thickening of the interval between the 

base of the Yellow Creek Tongue and the top of “A-groove,” 

from 90 ft in the Triangulation Station Shale core hole in sec. 

24, T. 7 S., R. 97 W. (fig. 4) to 1,000 ft in the U.S. Bureau of 

Mines/AEC Core hole No. 1 sec. 13, T. 1 N., R. 98 W. (Hail, 

1987). 

Dry Fork Tongue

The Dry Fork Tongue was named by Duncan and others 

(1974) from exposures in NE1⁄4 SE1⁄4 NW1⁄4 sec. 35, T. 1 

N., R. 97 W., along the Dry Fork of Piceance Creek. At the 

type locality it is 70 ft thick and consists of greenish-gray 

marlstone, silty marlstone, and calcareous siltstone with a 5- 

to 10-ft light-brown sandstone lens near the base. In the area 

where the tongue merges with the main body of the Green 

River Formation, it consists entirely of oil shale, including 

organic-rich beds 36 through 38. Where well developed, the 

sequence (labeled Tgd, fig. 4) has been called the Stillwater 

zone. Tongue Tgd has been mapped in parts of the White 

River City, Barcus Creek, Barcus Creek SE, Greasewood 

Gulch, and Segar Mountain quadrangles (fig. 5). The Dry Fork 

Tongue is underlain by and grades laterally into the Uinta 
10 Intertonguing in the Piceance Creek Basin During the Late Stages of Lake Uinta

Table 2. List of drill holes shown on cross sections.

Well name Location

U31 Skyline Oil Co., Watson 1 Sec. 5, T. 11 S., R. 25 E.

C20 Tell Ertl Jan Sec. 18, T. 2 S., R. 99 W.

C34 U.S. Bur. of Mines/AEC, Colorado 1 Sec. 13, T. 1 N., R. 98 W.

C35 U.S. Bureau of Mines/AEC, Colorado 2 Sec. 14, T. 1 S., R. 99 W.

C37 U.S. Bureau of Land Management, Triangulation Station Shale 2 Sec. 24, T. 7 S., R. 97 W.

C47 Union Oil Company, Florence Sec. 31, T. 4 S., R. 95 W.

C104 Gallagher - Victor, Gallagher Sec. 13, T. 6 S., R. 100 W.

C109 Pacific Oil Co., Allen 1 Sec. 34, T. 6 S., R. 97 W.

C111 Pacific Oil Co., Hardison 1 Sec. 2, T. 7 S., R. 98 W.

C124 Savage Oil Shale Development Corp., Hunter Sec. 23, T. 4 S., R. 98 W.

C126 Savage Oil Shale Development Corp., Camp Sec. 35, T. 4 S., R. 98 W.

C139 Getty Oil Co., 01–23/D Sec. 23, T. 5 S., R. 98 W.

C149 Mobil Oil Co., [Born] E (Core Hole 5) Sec. 11, T. 6 S., R. 96 W.

C150 Mobil Oil Co., [Born] F (Core Hole 6) Sec. 11, T. 6 S., R. 96 W.

C151 Marathon Oil Co., Square S No. 1 Sec. 4, T. 2 S., R. 97 W.

C172 Shell Oil Co. 23X–10 Sec. 10, T. 7 S., R. 97 W.

C183 Pure Oil Co., Battlement Mesa Core Hole Sec. 36, T. 7 S., R. 94 W.

C195 ARCO-Mobil-Equity, Figure Four 31–1 Sec. 31, T. 3 S., R. 98 W.

C254 ARCO(C–b Project), Sorghum Gulch 11 Sec. 7, T. 3 S., R. 96 W.

C262 Gulf/Std. Ind. (Ca Project) Ch 2–3 Sec. 33, T. 1 S., R. 99 W.

C272 Rio Blanco Oil Shale Co., C–14 Sec. 10, T. 2 S., R. 99 W.

C309 Shell Oil (Ertl Claims), 32–7 Sec. 7, T. 2 S., R. 99 W.

C310 Shell Oil (Ertl Claims), 42X–10 Sec. 10, T. 2 S., R. 100W.

C329 Carter Oil Co., Opportunity2 Sec. 28, T. 1 N., R. 96 W.

C330 Carter Oil Co., Square S No. 1 Sec. 21, T. 1 S., R. 97 W.

C333 Getty Oil Co., 32–27 Sec. 27, T. 5 S., R. 97 W.

C350 U.S. Navy, Core Hole 15/16 (NOSR I) Sec. 1, T. 5 S., R. 95 W.

C508 Phillips Petroleum Co., P–02 Sec. 23, T. 1 S., R. 100 W.

C517 Phillips Petroleum Co., P–19 Sec. 36, T. 1 S., R. 100 W.

C518 Phillips Petroleum Co., P–20 Sec. 26, T. 1 S., R. 100 W.
Description of Individual Tongues in the Green River and Uinta Formations 11

70

White River

R. 102 W. R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W. R. 94 W.

Colorado

64

139 64

13

Meeker

Rangely

Rio Blanco

139

13

Rifle

DeBeque

Parachute

Mesa

T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

T. 4 S.

T. 3 S.

T. 2 S.

T. 1 S.

T. 1 N.

T. 2 N.

R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W.

C139

C350

C111

C172

C149

C508

C262

C329

C310

C35

C330

C518

C124

C254

C333

G

G'

I

I'

C

E

A

A'

C'

E'

Index map

108°30' 108°00'

40°00'

39°30'

Highway

12 MILES

City or town

County line

Creek or river

0

Area underlain by

 the Mahogany bed

EXPLANATION

White River

GARFIELD

RIO BLANCO

Colorado River

Core hole

Figure 6. Locations of cross sections A–A′ (fig. 7), C–C′ (fig. 10), E–E′ (fig. 12), G–G′ (fig. 14), and I–I′ (fig. 16).
0 20 40 0 20 40 

200250

400450500 300

A 

C508 C262 

Phillips Petroleum Co., P–02 Sec. 23, T. 1 S., R. 100 W.

Gulf/Std. Ind. (Ca Project) Ch 2–3 Sec. 33, T. 1 S., R. 99 W.

300350450

BedBedBedBed

A-groove

Tggc tongue

Tu2

2

8

16

12

400

Oil yield, in gallons per ton

DE

DEPTH, IN

FEET

DEPTH, IN

FEET

Oil yield, in gallons per ton 

Figure 7. Cross section A–A′ showing stratigraphic relation of tongue Tggc (table 1) of the Green River Formation (blue) with associated tongues 

Formation (yellow). The underlying Uinta tongue is labeled Tu2. Individual oil shale beds labeled 2–16. Location of section shown in figure 6.
Description of Individual Tongues in the Green River and Uinta Formations 13

70

White River

R. 102 W. R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W. R. 94 W.

Colorado

64

139 64

13 White River

Meeker

Rangely

Rio Blanco

139

13

Rifle

DeBeque

Parachute

Mesa Colorado River

T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

T. 4 S.

T. 3 S.

T. 2 S.

T. 1 S.

T. 1 N.

T. 2 N.

Index map

C517

C309

C329

C20 C272

C151

C195

C126 C47

C104

C333

C150

C109

C37 C183

B

B'

D

D'

F

F'

H H'

J

J'

108°30' 108°00'

40°00'

39°30'

Highway

12 MILES

City or town

County line

Creek or river

0

Area underlain by

 the Mahogany bed

EXPLANATION

GARFIELD

RIO BLANCO

Core hole

Figure 8. Locations of cross sections B–B′ (fig. 9), D–D′ (fig. 11), F–F ′ (fig. 13), H–H′ (fig. 15), and J–J′ (fig. 17).
450 500 550 250 350 0 20 40 0 20 40 

B 

C517 C309 

Phillips Petroleum Co., P–19 Sec. 36, T. 1 S., R. 100 W. Shell Oil (Ertl Claims), 32–7 Sec. 7. T. 2 S., R. 99 W. 

Bed Bed Bed Bed Tgtc tongue 242220181612 300 550 450 Tu3 Bed 500 Oil yield, in gallons per ton Oil yield, in gallons per ton 

DEPTH, IN FEET DEPTH, IN FEET DEP

F

Figure 9. Cross section B–B′ showing stratigraphic relation of tongue Tgtc (table 1) of the Green River Formation (blue) with associated tongues 

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu3. Individual oil shale beds labeled 12–24. Location of section shown in figure 8.
Figure 10. Cross section C–C ′ showing stratigraphic relation of tongue Tgm (table 1) of the Green River Formation (blue) with associated tongue

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu4. Individual oil shale beds labeled 20–26. Location of section shown in figure 6.

DEPTH, IN

FEET

BedBedBedBedBed

300350400

800850

900

950

1,000

450500

550

25

24

22

20

26

0 20 40

0 20 0 

C 

C310 C35 

Shell Oil (Ertl Claims), 42x–10 Sec. 10, T. 2 S., R. 100 W.

U.S. Bureau of Mines/AEC, Colorado 2

 Sec. 14, T. 1 S., R. 99 W.

Carter O Sec. 

Tgm

tongue

Tu4

Oil yield, in gallons per ton Oil yield, in gallons per ton

Oil yie

DEPTH, IN

FEET

DEPTH, IN

FEET
200 150 100 350 400 450 350 400 D 

C272 Rio Blanco Oil Shale Co., C–14 Sec. 10, T. 2 S., R. 99 W. C20 Tell Ertl Jan Sec. 18, T. 2 S., R. 99 W. 

M 

0 20 40

0 20 40 

300

450

Tgy

tongue

Bed

BedBedBed

Tu5

36322625

Oil yield, in gallons per ton

Oil yield, in gallons per ton

DEPTH, IN

FEET

DEPTH, IN

FEET

DEPTH

FEET

Figure 11. Cross section D–D′ showing stratigraphic relation of tongue Tgy (table 1) of the Green River Formation (blue) with associated tongues 

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu5. Individual oil shale beds labeled 25–36. Location of section shown in figure 8.
100150200250

0 20

150

0 20 40

900950

0 

100

850

Bed

BedBed

Bed

Bed

44

40

48

46

38

E 

C518 C124 

Phillips Petroleum Co., P–20 Sec. 26, T. 1 S., R. 100 W.

Savage Oil Shale Development Corp., Hunter Sec. 23, T. 4 S., R. 98 W.

ARCO  

Tu7

Tu7

Tgt

tongue

Oil yield, in gallons per ton

Oil yield, in gallons per ton O

DEPTH, IN

FEET

DEPTH, IN

FEET

DEPTH, 

FEET

Figure 12. Cross section E–E ′ showing stratigraphic relation of tongue Tgt (table 1) of the Green River Formation (blue) with associated tongues o

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu7. Individual oil shale beds labeled 38–48. Location of section shown in figure 6.
Bed

550600650

150200250

300

350

100150

BedBed

Bed

54

524850

40

0 20 40 60 0 20 40

0 

F 

C195 C126 

ARCO–Mobil–Equity, Figure Four 31–1 Sec. 31, T. 3 S., R. 98 W.

Savage Oil Shale Development Corp., Camp Sec. 35, T. 4 S., R. 98 W.

Tgb

tongue

Tu8

Oil yield, in gallons per ton Oil yield, in gallons per ton 

DEPTH, IN

FEET

DEPTH, IN

FEET

DEPTH, IN

FEET

Figure 13. Cross section F–F ′ showing stratigraphic relation of tongue Tgb (table 1) of the Green River Formation (blue) with associated tongues 

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu8. Individual oil shale beds labeled 40–54. Location of section shown in figure 8.
400450500

650700

750

Bed

Bed

Bed

BedBed

58

60

56

54

52

800

850

0 20 40 0 20 40 

G 

C139 C333 

 Getty Oil Co., 01-23/DSec. 23, T. 5 S., R. 98 W.

 Getty Oil Co., 32-27Sec. 27, T. 5 S., R. 97 W.

U 

750

Tgc

tongue

Tu9

Oil yield, in gallons per ton Oil yield, in gallons per ton 

DEPTH, IN

FEET DEPTH, IN

FEET DEPTH

FEE

Figure 14. Cross section G–G′ showing stratigraphic relation of tongue Tgc (table 1) of the Green River Formation (blue) with associated tongues 

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu9. Individual oil shale beds labeled 52–60. Location of section shown in figure 6.
Figure 15. Cross section H–H′ showing stratigraphic relation of tongue Tgs (table 1) of the Green River Formation (blue) with associated tongues 

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu10. Individual oil shale beds labeled 58–64. Location of section shown in figure 8

100

50

0 20 40 0 20 40 0 

Bed

Bed

Bed

Bed

550

600

650

700

350

400

450

64

62

60

58

Tgs

tongue

H 

C104 C333 

Gallagher – Victor, Gallagher Sec. 13, T. 6 S., R. 100 W.

 Getty Oil Co., 32–27Sec. 27, T. 5 S., R. 97 W.

Mobil Oil  Sec

Tu10

Oil yield, in gallons per ton Oil yield, in gallons per ton Oil yie

DEPTH, IN

FEET

DEPTH, IN

FEET

DEPTH, IN

FEET
Figure 16. Cross section I–I′ showing stratigraphic relation of tongue Tgj (table 1) of the Green River Formation (blue) with associated tongues of 

Formation (yellow). The underlying Uinta tongue is labeled Tu12. Individual oil shale beds labeled 62–72. Location of section shown in figure 6.

100150200

5560

650

200150

Bed

Bed

BedBed

Porcupine Creek Tuff 

72

6864

62

I 

C111 C172 

Pacific Oil Co., Hardison 1 Sec. 2, T. 7 S., R. 98 W.

 Shell Oil Co., 23X–10Sec. 10, T. 7 S., R. 97 W.

0 20 40

0 20 40 60 80

Tgj

tongue

Tu12

Oil yield, in gallons per ton

Oil yield, in gallons per ton

DEPTH, IN

FEET DEPTH, IN

FEET

DEPT

FE
Figure 17. Cross section J–J ′ showing stratigraphic relation of tongue Tgsk (table 1) of the Green River Formation (blue) with associated tongues 

Uinta Formation (yellow). The underlying Uinta tongue is labeled Tu13. Individual oil shale beds labeled 70–76. Location of section shown in figure 8

0 20 40

0 20 40 60

0 

100150200

Bed

BedBed

200250

Porcupine Creek Tuff

76

72

70

50

100

150

J 

C109 C37 

 Pacific Oil Co., Allen 1Sec. 34, T. 6 S., R. 97 W.

U.S. Bureau of Land Management, Triangulation Station Shale 2 Sec. 24, T. 7 S., R. 97 W.

Pure Oi 

Tgsk

tongue

Tu13

Oil yield, in gallons per ton

Oil yield, in gallons per ton

O

DEPTH, IN

FEET

 DEPTH, IN

FEET

DEPTH, IN

FEET
Description of Individual Tongues in the Green River and Uinta Formations 23

tongue Tu6 (fig. 4); a cross section was not prepared for this 

report because of the lack of adequate data.

Thirteenmile Creek Tongue 

The Thirteenmile Creek Tongue (Tgt, fig. 4; cross 

section E–E′, figs. 6 and 12) was named by Duncan and others 

(1974) from exposures on the north side of Dry Thirteenmile 

Creek in the SW1⁄4 sec. 26, SE1⁄4 sec. 27 and NW1⁄4 sec. 34, 

T. 2 S., R. 95 W. At the type locality it is 137 ft thick and 

consists mostly of marlstone and oil shale with a 25-ft lightbrown sandstone near the top. Where the tongue joins the 

main body of the Green River Formation, it consists entirely 

of oil shale that includes three organic-rich beds 40, 42, and 

44 (fig. 4) that have been named the “Big Three.” Tongue 

Tgt has been mapped throughout most of the Greasewood 

Gulch quadrangle and in parts of the White River City, Barcus 

Creek SE, Barcus Creek, Wolf Ridge, Square S Ranch, Segar 

Mountain, No Name Ridge, Jessup Gulch, and Rock School 

quadrangles (fig. 5). It extends in the subsurface into the Bull 

Fork quadrangle and merges with the main body of the Green 

River Formation in the northeast corner of the No Name 

Ridge quadrangle and in the subsurface in the Jessup Gulch 

and Rock School quadrangles. Uinta tongue Tu7 consists 

of the sandstone beds that underlie and intertongue with the 

Thirteenmile Creek Tongue (fig. 12). 

Black Sulphur Tongue 

The Black Sulphur Tongue was named by Duncan and 

others (1974) from exposures along Black Sulphur Creek 

in NW1⁄4 SW1⁄4 SW1⁄4 sec. 23, T. 2 S., R. 98 W., in the Rock 

School quadrangle (fig. 5). At the type locality, the tongue 

(labeled Tgb, fig. 4; cross section F–F′, figs. 8 and 13) is 

30 ft thick and consists mainly of gray and light-brown silty 

marlstone with a 6-ft siltstone bed 18 ft below the top. In the 

area, close to its join with the main body of the Green River 

Formation, it consists entirely of oil shale, including organicrich oil shale beds 48 through 52 (fig. 13). Where a sandstone 

bed separating oil shale bed 52 from bed 54 in some places 

is missing, bed 54 is mapped as the uppermost unit in tongue 

Tgb. The unit has been mapped in the Wolf Ridge, Square 

S Ranch, Jessup Gulch, Rock School, and Yankee Gulch 

quadrangles (fig. 5). Although not mapped, it extends into 

the Black Cabin Gulch quadrangle and may be traced in the 

subsurface into the McCarthy Gulch, Bull Fork, Jessup Gulch, 

and Figure Four Spring quadrangles (fig. 5). The tongue 

grades into the Uinta Formation in the Greasewood Gulch 

and Segar Mountain quadrangles and merges with the main 

body of the Green River Formation in the No Name Ridge 

quadrangle. Uinta tongue Tu8 includes the sandstone at the 

base of oil shale bed 48 and other intervening sandstones 

between bed 48 and bed 54 (fig. 4).

Coughs Creek Tongue 

The Coughs Creek Tongue was named by O’Sullivan 

(1975) from exposures along the west side of Cow Creek 

about a mile northwest of the junction of Coughs Creek with 

Cow Creek in the SW1⁄4 SE1⁄4 sec. 12, T. 4 S., R. 95 W., in 

the Rio Blanco quadrangle (fig. 5). The tongue (Tgc, fig. 4; 

cross section G-G′, figs. 6 and 14) at the type locality consists 

of an upper marlstone unit 23 ft thick, a middle tan siltstone, 

sandstone, and gray marlstone unit 55 ft thick, and a lower 

thin-bedded gray marlstone unit 30 ft thick, with a 0.1-ft-thick 

rich oil shale bed in the middle. In the area near the join of 

the tongue with the main body of the Green River Formation, 

it consists entirely of oil shale (beds 54 through 58, fig. 14). 

In some localities, the sandstone bed between beds 52 and 54 

is thin or missing, in which case bed 54 has been mapped as 

part of the Black Sulphur Tongue. The Coughs Creek Tongue 

has been mapped throughout most of the Figure Four Spring, 

Cutoff Gulch, and McCarthy Gulch quadrangles and in parts 

of the Thirteen Mile Creek, No Name Ridge, Rio Blanco, 

Forked Gulch, Circle Dot Gulch, Mount Blaine, and Bull Fork 

quadrangles (fig. 5). Although not mapped, it probably extends 

into the Jessup Gulch, Razorback Ridge, Yankee Gulch, and 

Rock School quadrangles. The tongue merges with the main 

body of the Green River Formation in the Rio Blanco, Forked 

Gulch, Circle Dot Gulch, and Mount Blaine quadrangles 

and grades into the Uinta Formation in the No Name Ridge 

quadrangle and probably in the Jessup Gulch, Rock School, 

and Yankee Gulch quadrangles. Uinta tongue Tu9 includes 

the sandstone below bed 54 and all intervening sandstones 

between beds 54 and 58.

Stewart Gulch Tongue 

The Stewart Gulch Tongue of the Green River Formation 

(Tgs, fig. 4; cross section H–H′, figs. 8 and 15) was named 

by Hail (1977) from exposures on the east side of the Middle 

Fork of Stewart Gulch in the SW1⁄3 NE1⁄3 NW1⁄4 sec. 3, T. 4 S., 

R. 96 W., in the Cutoff Gulch quadrangle. At the type locality, 

the tongue consists of an upper unit of 23 ft of light-gray to 

light-brownish gray silty marlstone, with 1 ft of light-brown 

oil shale near the middle, underlain by 10 ft of light-brown 

sandstone, 13 ft of greenish-gray marly siltstone, and 4 ft of 

light-brown silty marlstone. In the area near its join with the 

main body of the Green River Formation, the tongue consists 

mostly of oil shale, including beds 60 and 62 (figs. 4 and 

15). It is present throughout the Bull Fork and Cutoff Gulch 

quadrangles, merges with the main body of the Green River 

Formation in the McCarthy Gulch, Forked Gulch, Circle Dot 

Gulch, Mount Blaine, and Figure Four Spring quadrangles, 

and, although not mapped, extends into and possibly grades 

into the Uinta Formation in the Razorback Ridge, Rock 

School, Jessup Gulch, and No Name Ridge quadrangles 

(fig. 5). The Tu10 tongue of the Uinta Formation includes 
24 Intertonguing in the Piceance Creek Basin During the Late Stages of Lake Uinta

the sandstone bed at the base of oil shale bed 60 and all other 

sandstone beds between beds 60 and 62. 

Marlstone at Barnes Ridge and Marker Bed at 

Bull Fork

The “Marlstone at Barnes Ridge” and “Marker Bed at 

Bull Fork” are generally exposed as white bands on ridges 

that cap the Roan Plateau and trend westward to Ts. 4 and 5 S. 

Hail (1975, 1977) described the Barnes Ridge unit in the Cutoff Gulch quadrangle, and the Bull Fork unit in the Bull Fork 

quadrangle as consisting of gray to light-brown marlstone 

with thin beds of oil shale. No assays of the marlstones are 

available and the sandstone and siltstone beds underlying the 

units were not specifically identified, so no cross section was 

prepared for this report to illustrate their intertonguing relation 

with the Uinta Formation. Near the merger of these units with 

the main body of the Green River Formation, they consist 

almost entirely of oil shale (possibly beds 64 and 66, fig. 4). 

The Barnes Ridge unit is mapped throughout much of the 

Cutoff Gulch and Bull Fork quadrangles and in the southeast 

quarter of the Figure Four Spring quadrangle (fig. 5). It has 

been mapped as Tgs1 in places along the northern boundary 

of the Mount Blaine quadrangle and as Tgj along the northern 

boundary of the Circle Dot Gulch quadrangle and the western 

boundary of the McCarthy Gulch quadrangle. The unit merges 

with the Stewart Gulch Tongue (Tgs, fig. 4) in the southeast 

quarter of the Figure Four Spring quadrangle. The Bull Fork 

is mapped high on the ridges in the south half of the Bull Fork 

quadrangle. At places along the northern boundary of the 

Mount Blaine quadrangle it is also mapped as Tgsl.

Marlstone at Sleepy Ridge and Marlstone at 

Jackrabbit Ridge 

The “Marlstone at Sleepy Ridge” was named by Hail 

(1978) from exposures on Sleepy Ridge in the Mount Blaine 

quadrangle (fig. 5). The approximate stratigraphic equivalent 

in the Circle Dot Gulch quadrangle was named the “Marlstone 

at Jackrabbit Ridge” by Hail (1992) from exposures on 

Jackrabbit Ridge. At the type locality, the Sleepy Ridge and 

the Jackrabbit Ridge units (Tgsl and Tgj, fig. 4; cross section 

I–I′, figs. 6 and 16) both consist of light-gray marlstone, 

oil shale, siltstone, and sandstone. Near the area where they 

merge with the main body of the Green River Formation they 

are mainly oil shale (beds 64 through 70, fig. 4).

The “Marlstone at Sleepy Ridge” is mapped almost 

throughout the Mount Blaine quadrangle, as well as in the 

north half of the Desert Gulch quadrangle (fig. 5) where it 

merges with the main body of the Green River Formation. The 

“Marlstone at Jackrabbit Ridge” is mapped in the Circle Dot 

Gulch, McCarthy Gulch, Rio Blanco, Forked Gulch, and Anvil 

Points quadrangles. It grades into the Uinta Formation in the 

southern part of the No Name Ridge quadrangle and merges 

with the main body of the Green River Formation in the 

middle of the Circle Dot Gulch quadrangle, the northern third 

of the Forked Gulch quadrangle, and the northern half of the 

Anvil Points quadrangle; it may also be traced in the subsurface into the Red Pinnacle and Long Point quadrangles. The 

Tul2 tongue of the Uinta Formation includes the sandstone 

bed at the base of bed 64 and all other sandstone beds between 

64 and 70 (fig. 4).

Marlstone at Skinner Ridge 

The uppermost tongue of the Green River Formation 

in Colorado is the “Marlstone at Skinner Ridge,” named by 

Hail (1978) for exposures on Skinner Ridge in the Mount 

Blaine quadrangle (fig. 5). At the type locality, the unit (Tgsk, 

fig. 4; cross section J–J ′, figs. 8 and 17) consists of light-gray 

marlstone and silty marlstone with some lean oil shale, and 

in places contains thin papery oil shale beds. It is separated 

from the marlstone at Sleepy Ridge by a tuff or tuffaceous 

sandstone bed informally named the “Porcupine Creek Tuff” 

(Uinta tongue Tu13, fig. 4) from exposures in the drainage of 

Porcupine Creek in the North Mamm Peak quadrangle on the 

north flank of Battlement Mesa. This tuff bed is believed to 

be the stratigraphic equivalent of the Horse Bench Sandstone 

Bed in the eastern part of the Uinta Basin, Utah. In the area 

near where the Skinner Ridge merges with the main body 

of the Green River Formation it consists mainly of oil shale 

(beds 72 through 76, fig. 4). The “Marlstone at Skinner Ridge” 

has been mapped in the Mount Blaine and Desert Gulch 

quadrangles (fig. 5). It is probably present, but not mapped, 

in the Red Pinnacle, Long Point, The Saddle, Anvil Points, 

Rulison, North Mamm Peak, and Hawxhurst quadrangles. The 

Hawxhurst and North Mamm Peak quadrangles are adjacent 

to the Rulison quadrangle on the south and east and are not 

shown on figure 5.

At places in the Piceance Creek Basin there are oil shale 

beds that are stratigraphically higher than the beds in the 

marlstone at Skinner Ridge. They are in lenses of limited areal 

extent and were probably deposited in isolated ponds during 

late phases of Lake Uinta.

Shale-Oil Resources

The shale-oil resource estimates by the USGS—about 

1 trillion barrels of oil contained in the Green River Formation in the Piceance Creek Basin—included resources from a 

series of seven rich oil shale zones and the intervening leaner 

zones (Pitman and others, 1989). These figures, however, do 

not include the sequence of moderately rich oil shale beds, 

between the “A-groove” and the top of the Big 3 (beds 2 

through 44, fig. 4), commonly called the R-8 zone. The R-8 

zone was estimated to contain a total of 128.5 billion barrels of 

oil based mostly on assay data from cores drilled prior to 1967 

(Pitman and Donnell, 1973). The zone, in an area of about 
References Cited 25

400 mi2 within Ts. 4, 5, and 6 S. and Rs. 95 to 98 W., is free of 

clastic interbeds of the Uinta Formation. The zone in this area 

averages about 250 ft in thickness, has an average value of 

16 gallons of oil per ton of shale, and is estimated to contain 

an in-place resource of 73 billion barrels of oil. As noted by 

Pitman and Donnell (1973) the R-8 zone is richest in T. 5 S., 

R. 96 W., where it is 250 ft thick and has an average yield of 

19 gallons of oil per ton with a total in-place resource of 6.5 

billion barrels.

Shale-oil resources published by Pitman and Donnell 

(1973) were reported in four separate units that may be 

equated in general to one or more combinations of the Green 

River tongues described in this report, as follows: Unit 1 

is equivalent to tongues Tggc, Tgtc, and Tgm; Unit 2 is 

equivalent to tongue Tgy; and Units 3 and 4 are equivalent to 

tongues Tgd and Tgt.

References Cited

Donnell, J.R., 1982, Preliminary geological map of the 

Sagebrush Hill quadrangle, Rio Blanco County, Colorado: 

U.S. Geological Survey Miscellaneous Field Studies Map 

MF–1398, scale 1:24,000.

Duncan, D.C., Hail, W.J., O’Sullivan, R.B., and Pipiringos, 

G.N., 1974, Four newly named tongues of Eocene Green 

River Formation, Northern Piceance Creek basin, Colorado: 

U.S. Geological Survey Bulletin 1394–F, 13 p. 

Franczyk, K.J., Fouch, T.D., Johnson, R.C., Molenaar, C.M., 

and Cobban, W.C., 1992, Cretaceous and Tertiary paleographic reconstructions for the Uinta-Piceance basin study 

area Colorado and Utah: U.S. Geological Survey Bulletin 

1787–Q, 37 p.

Hail, W.J., Jr., 1975, Preliminary geologic map of the Cutoff 

Gulch quadrangle, Rio Blanco and Garfield counties, 

Colorado: U.S. Geological Survey Miscellaneous Field 

Studies Map MF–691, scale 1:24,000.

Hail, W.J., Jr., 1977, Stewart Gulch Tongue—A new tongue of 

the Eocene Green River Formation, Piceance Creek Basin, 

Colorado: U.S. Geological Survey Bulletin 1422–E, 8 p.

Hail, W.J., Jr., 1978, Preliminary geologic map of the Mount 

Blaine quadrangle, Garfield County, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF–984, scale 1:24,000.

Hail, W.J., Jr., 1984, Geologic map of the Barcus Creek 

quadrangle, Rio Blanco County, Colorado: U.S. Geological 

Survey Geologic Quadrangle Map GQ–1578, 

scale 1:24,000.

Hail, W.J., Jr., 1987, Chart showing intertongued units of the 

Eocene Green River and Uinta Formations, northwestern 

Piceance Creek Basin, northwestern Colorado: U.S. 

Geological Survey Miscellaneous Series Map I–1797–B.

Hail, W.J., Jr., 1990, Geology of the lower Yellow Creek area, 

northwestern Colorado: U.S. Geological Survey Bulletin 

1787–O, 45 p.

Hail, W.J., Jr., 1992, Geology of the central Roan Plateau area 

northwestern Colorado: U.S. Geological Survey Bulletin 

1787–R, 26 p., 2 pls.

O’Sullivan, R.B., 1974, Chart showing correlation of selected 

units of the Eocene Uinta and Green River Formations, eastcentral Piceance Creek basin, northwestern Colorado: U.S. 

Geological Survey Oil and Gas Investigations Chart OC–67.

O’Sullivan, R.B., 1975, Coughs Creek Tongue—A new tongue 

of the Eocene Green River Formation, Piceance Creek 

basin, Colorado: U.S. Geological Survey Bulletin 1395–G, 

7 p.

O’Sullivan, R.B., 1987, Chart showing correlation of selected 

parts of the Eocene Uinta and Green River Formations, 

southeastern Piceance Creek basin, Colorado: U.S. 

Geological Survey Field Studies Map MF–1986.

Pitman, J.K., and Donnell, J.R., 1973, Potential shale-oil 

resources of a stratigraphic sequence above the Mahogany 

zone, Green River Formation, Piceance Creek Basin, 

Colorado: U.S. Geological Survey Journal of Research, v. 1, 

no. 4, p. 467–473.

Pitman, J.K., and Johnson, R.C., 1978, Isopach, structure 

contour and resource maps of the Mahogany oil-shale zone, 

Green River Formation, Piceance Creek basin, Colorado: 

U.S. Geological Survey Field Studies Map MF–958, 

scale 1:126,720.

Pitman, J.K., Pierce, F.W., and Grundy, W.D., 1989, 

Thickness, oil yield, and kriged resource estimates for 

the Eocene Green River Formation, Piceance Creek 

basin, Colorado: U.S. Geological Survey Oil and Gas 

Investigations Chart OC–132.
26 Intertonguing in the Piceance Creek Basin During the Late Stages of Lake Uinta

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